Revert to 2.1.2
    
    We revert back to 2.1.2 as it's the current version in
    u-c currently [1].
    The mistake is due to the requirements being cut, so
    consequently Xena and Yoga have diverged.
    
    [1] https://github.com/openstack/requirements/blob/stable/xena/upper-constraints.txt
    
    Related-to: https://review.rdoproject.org/r/q/I70808e6cb45b763822addd1ba3de3e4b87e708c5